1. Field of the Invention
This invention relates to multi-base propellants, and especially to cross-linked plastisol propellants suited for use in rocket motor assemblies. This invention further relates to rocket motor assemblies loaded with the multi-base propellants.
2. Description of the Related Art
A typical solid fuel rocket motor generally comprises a case of metal or reinforced polymeric composite material and a nozzle attached to the case. Within the case is a propellant grain, which upon firing undergoes combustion reactions to generate large quantities of combustion gases and particles (i.e., combustion products). The combustion products generated by the propellant grain are expelled through the nozzle attached to the case. Nozzles are designed to accelerate the combustion product gases from the propellant grain to the maximum velocity at exit. Most commonly, this design involves a provision in the nozzle pathway comprising a throat having a restricted cross-sectional area, and a frustoconical skirt aft of the throat. The throat and skirt collectively define a converging/diverging configuration to the nozzle pathway. A heat insulating layer and a liner are usually interposed between the grain and the outer case to protect the outer case from the high operating temperatures associated with rocket motor operation and the erosive high velocity particles generated during combustion of the propellant grain. The liner serves the additional function of enhancing grain-to-case or grain-to-insulation bonding.
Propellants containing nitrocellulose as the principle energetic polymeric binder plasticized with one or more plasticizers are commonly referred to in the art as double-base propellants. A typical formulation for a double-base propellant includes, as its main ingredients, 10-90 wt % nitrocellulose and 10-90 wt % plasticizer, more preferably 40-70 wt % nitrocellulose and 30-60 wt % plasticizer. Among the plasticizers most commonly used in the art for forming double-base propellants are nitroglycerine, butanetrioltrinitrate, and diglycol dinitrate.
Another common ingredient used with plasticized nitrocellulose-based propellants is nitroguanidine. Propellants containing nitrocellulose, one or more plasticizers, and nitroguanidine are commonly referred to in the art as a triple-base propellant. (The term triple-base propellant has also sometimes been used to denote propellants containing nitrocellulose, one or more plasticizers, and energetic fuels other than nitroguanidine.) It is common in the art to classify both double-base and triple-base propellants as multi-base propellants.
Another class of propellants is composite-modified multi-base propellants, in which the nitrocellulose serves the additional function of acting as a binder to immobilize oxidizer particles (e.g., ammonium perchlorate) and/or fuel (e.g., aluminum) particles.
It is known in the art to make multi-base propellants from plastisol-grade nitrocellulose. The term xe2x80x9cpelletized nitrocellulosexe2x80x9d (PNC) propellant refers to multi-base propellants made via a conventional slurry mixing technique in which the pelletized nitrocellulose is processed by slurry mixing and pouring the mixed slurry, in an uncured state, into casting molds or rocket motors in a casting step. The slurry is prepared by dispersing pelletized nitrocellulose having diameters generally on the order of 1 to 20 microns in a suitable non-solvent diluent, most commonly heptane. To the slurry is added a suitable nitrate ester plasticizer, such as nitroglycerin and/or butanetrioltrinitrate (BTTN). Other processing agents and chemical stabilizers, such as N-methyl-p-nitroaniline (MNA), are also added to the slurry at this stage. After removing a portion of the heptane from the top of the formulation, mixing is performed under vacuum conditions to remove remnants of the heptane from the slurry. Next, further ingredients are added and the formulation is mixed in an appropriate mixer, such as a vertical mixer. These ingredients include, among others, fibers, ballistic additives, energetic solid fuels, and, in the case of a composite multi-base propellant, oxidizer particles and/or fuel particles. After thoroughly mixing the formulation, a suitable cross-linker (e.g., a diisocyanate) may be added and the propellant is cast and cured to form a homogenous propellant.
Advantageous properties associated with multi-base propellants include their excellent ambient mechanical properties, low shock sensitivity, excellent ballistics, and operational characteristics, as well as their low signature plumes. These properties make multi-base propellants highly desirable for many rocket motor applications. However, the use of multi-base propellants is not without its problems.
Several hazards and time-consuming steps make the conventional plastisol production process undesirable for large-scale implementation. For example, although the pelletized nitrocellulose is relatively safe to handle when diluted in heptane, without the diluent the dry nitrocellulose is extremely sensitive to electrostatic discharge (ESD), especially prior to admixture of the nitrocellulose with plasticizer. The ESD sensitivity of the nitrocellulose is especially problematic with nitrocellulose in dry pellet form, since the pellets are characterized by a relatively high surface area. During normal handling of heptane-wet pelletized nitrocellulose, the heptane tends to evaporate due to its low boiling point. Evaporation of the heptane from the slurry tends to leave small quantities of hazardous (electrostatic-discharge sensitive) dry nitrocellulose on the surfaces of tooling and bulk container walls. Special precautions must be taken to avoid the deposition of hazardous dry nitrocellulose and, when such precautions are not fully effective and dry nitrocellulose is deposited on the tooling and bulk container walls, to safely remove the dry nitrocellulose without incident. Removal of the heptane diluent from the plasticized slurry during processing is also labor-intensive, time-consuming, and is usually performed at various stages of the conventional process, requiring repeated assaying of heptane concentration. Heptane is a low conductivity, flammable and hazardous solvent, and must be handled with caution.
Additionally, despite the excellent mechanical properties that multi-base propellants possess at ambient temperatures, multi-base propellants have consistently been found to exhibit inferior mechanical properties, such as tensile strength, at extreme low and elevated temperatures. Dramatic temperature changes that a multi-base propellant experiences in normal fabrication and use may generate mechanical strain in the propellant. If the multi-base propellant does not have satisfactory mechanical properties, these mechanical strains may increase the likelihood of fracture to the propellant grain, especially at low temperature ignition. Fractures in a propellant grain can, if widespread, significantly increase the propellant surface area available for combustion reaction. Attempting to anticipate the degree of fracture and the locations at which fractures will occur adds a large degree of uncertainty and unpredictability to motor performance. As a consequence, the chamber pressure created during combustion of a multi-base propellant grain can be increased to unanticipated levels.
The present invention is directed to a method of making a multi-base propellant by a suitable technique that substantially avoids the hazards and deleterious processing economies associated with the formation of dry nitrocellulose on processing equipment and tooling, yet produces a multi-base propellant that is mechanically robust, even over a wide range of operating temperatures such as xe2x88x9246xc2x0 C. (xe2x88x9250xc2x0 F.) to 66xc2x0 C. (150xc2x0 F.), which are normally experienced in rocket motor operation.
In accordance with the principles of this invention, a method for making multi-base propellants according to one embodiment of the invention in which pelletized nitrocellulose is coated with an electrostatically insensitive liquid elastomer precursor while wetted in an appropriate non-solvent diluent (e.g., an alkane such as heptane), in the absence of plasticizers, is provided. The non-solvent diluent is then substantially, if not completely, removed from the coated nitrocellulose. Subsequent to removing the diluent, the coated pelletized nitrocellulose is mixed with one or more plasticizers and optionally other ingredients and fillers, including (optionally) energetic fuels such as nitroguanidine for making triple-base propellants. In the event that a composite-modified multi-base propellant is desired, oxidizer particles and fuel particles are also added to and mixed with the coated nitrocellulose. The propellant formulation is then cast, typically into a rocket motor case or a mold of suitable configuration. If a cross-linked multi-base propellant is desired, the cast propellant formulation is then cured with an acceptable curative, such as a diisocyanate or polyisocyanate, which is preferably added with the other optional ingredients and fillers prior to casting. The resulting material is visually (i.e., to the naked eye) homogeneous, insofar as there are no discrete nitrocellulose pellets or particle-like formations remaining in the cured propellant. Also, the coated nitrocellulose pellets present during processing have reduced sensitivity to electrostatic discharge.
Unlike the conventional process in which the pelletized nitrocellulose is not coated during diluent evaporation and can deposit as dry nitrocellulose powder on tooling and bulk container walls, in the novel process of this invention the nitrocellulose is coated with a liquid elastomer precursor prior to removal of the diluent. As a consequence, upon removal of the diluent, any nitrocellulose that deposits on tooling and bulk container walls is coated with a protective coating, which shields the pelletized nitrocellulose from the influences of electrostatic discharges.
The present invention provides a novel method in which most, if not all, of the organic non-solvent (e.g., an alkane such as heptane) can be removed in a single step, such as by heating, prior to adding the plasticizer. As a result, the inventive method avoids the need for repeated diluent removal and assaying steps. The inventors have discovered that by obviating the need for repeated diluent removal and assaying steps, an uncured propellant formulation can be made in accordance with the inventive process in approximately 50% to 80% of the amount of time needed to practice the conventional method. Substantial savings in operating costs and time and manpower can be realized by this reduction in processing time.
The present invention further provides a method of making a cured multi-base propellant, especially a minimum smoke Class 1.3 propellant, that contains a dispersed elastomer and is visually homogenous, insofar as no discrete nitrocellulose pellets or particle-like remnants remain in the propellant subsequent to curing. This may be achieved by practicing the method described above, although the visually homogeneous cured multi-base propellant described herein is not limited to propellants made by this embodiment.
Still further, the present invention provides a method for making multi-base propellants in which pelletized nitrocellulose is coated with an electrostatically insensitive liquid non-plasticizer while wetted in an appropriate non-solvent diluent (e.g., an alkane such as heptane), in the absence of plasticizers. As referred to in the context of this embodiment of the invention and as understood in the art, non-plasticizer means a liquid that does not swell the nitrocellulose, and is not meant to encompass the elastomer precursors described above. Representative non-plasticizers include n-alkyl citrate (e.g., CITROFLEX), diethyl suberate, diethyl sebacate, di-n-propyl adipate, IDP (isodecylperlargonate), and combinations thereof. Other non-plasticizers that are believed to be suitable include, by way of example, DOA (dioctyladipate), DOP (dioctylphthalate), DOM (dioctylmaleate), DBP (dibutylphthalate), diethylphthalate, dipropylphthalate, diethyl pimelate, and combinations thereof. The non-solvent diluent is then substantially, if not completely, removed from the non-plasticizer-coated nitrocellouse. Subsequent to removing the diluent, the coated pelletized nitrocellulose is mixed with one or more plasticizers and optionally other ingredients and fillers, including (optionally) energetic fuels such as nitroguanidine for making triple-base propellants. In the event that a composite-modified multi-base propellant is desired, oxidizer particles and fuel particles are also added to and mixed with the coated nitrocellulose. The propellant formulation is then cast, typically into a rocket motor case or a mold of suitable configuration. If a cross-linked multi-base propellant is desired, the cast propellant formulation is then cured with an acceptable curative, such as a diisocyanate or polyisocyanate, which is preferably added with the other optional ingredients and fillers prior to casting. Although not wishing to be bound by any theory, it is believed that the isocyanate moieties of the curative react with the hydroxide groups of the nitrocellulose. The resulting material may be visually (i.e., to the naked eye) homogeneous, insofar as there are no discrete nitrocellulose pellets or particle-like formations remaining in the cured propellant. Also, the coated nitrocellulose pellets present during processing have reduced sensitivity to electrostatic discharge.
This invention is also directed to coated nitrocellulose pellets, rocket motor assembles comprising solid multi-base propellants derived from the coated nitrocellulose pellets, and to a method of making the rocket motor assemblies.